The present invention relates in general to an apparatus and method for measuring the level of molten metal in an electromagnetic continuous casting process, and more particularly to an apparatus and method for measuring the level of the surface of molten metal in an electromagnetic continuous casting process using an alternating current (AC) electromagnetic field.
As well known to those skilled in the art, an electromagnetic continuous casting process is a technique for applying an electromagnetic field across the surface of molten metal and casting the molten metal using an electromagnetic force and Joule heat induced due to the applied electromagnetic field. In this technique, a part of the molten metal to be initially solidified is heated by the Joule heat and then slowly cooled, resulting in the formation of a thin, initial solid shell under the surface of the molten metal, thereby making it difficult to form an oscillation mark (OM). Further, because a contact angle between a casting mold and the molten metal surface is increased due to the electromagnetic force, the initial solid shell is less influenced by a mold oscillation, resulting in an improvement in surface characteristic of a cast product. On the other hand, such an electromagnetic continuous casting process is adapted to apply a strong AC electromagnetic field across a free surface (referred to hereinafter as surface) of the molten metal differently from existing processes, which field is induced by applying AC current to an induction coil installed in a place outside the casting mold where the molten metal surface is to be positioned.
In the above-mentioned electromagnetic continuous casting process, the surface of the molten metal is an important factor in determining the surface state of a cast product and must thus be finely controlled to maintain the surface state of the cast product better. For this reason, it is very important to accurately measure the surface level of the molten metal. In the case where the surface of the molten metal is beyond a desired level, a remarkable OM is formed on the surface of a continuous cast product, which may lead to a serious defect in the subsequent process. In this regard, the surface level of the molten metal in the electromagnetic continuous casting process must be more strictly managed than in general casting processes. It is generally known in the art that the surface characteristic of a cast product is most excellent when the surface of the molten metal has the same height as that of the top end of the induction coil.
For measuring the surface level of molten metal in existing steel continuous casting processes, there have conventionally been proposed a method using radioactive rays incapable of being transmitted through the molten metal, a method using an eddy current sensor (see U.S. Pat. No. 4,567,435, 1987), a method using an electrostatic capacity sensor (see U.S. Pat. No. 4,555,941, 1985), etc. However, these methods cannot be used for the electromagnetic continuous casting process where a strong AC electromagnetic field is present on the surface area of the molten metal, because their measuring devices are subjected to heating or restrictions in space. The method based on the eddy current sensor is desirable to finely measure the surface level of the molten metal, but disadvantageous in that the eddy current sensor is positioned within a mold. Namely, when a strong electromagnetic field is applied to the surface area of the molten metal as in the electromagnetic continuous casting process, the eddy current sensor is magnetically saturated and thus loses its function as the sensor. The method based on the electrostatic capacity sensor is subjected to a strict restriction in space because no conductive material having an effect on electric potential must be present within the range of a distance between an electrode and the surface of the molten metal. Further, because the electrostatic capacity sensor is considerably influenced by a dielectric constant of powder in the surface of the molten metal in casting the molten metal, its output varies with the thickness of the powder, resulting in the generation of an error within a very wide range. Moreover, a conductive material, which has to be installed relatively near the surface of the molten metal, may be induction-heated by the strong electromagnetic field in the electromagnetic continuous casting, thereby causing it not to act as the sensor. Similarly, in the method based on the radioactive rays, a proposed device cannot perform its own function due to the induction heating by the electromagnetic field.
For measuring the surface level of molten metal in the electromagnetic continuous casting process, there have conventionally been available a method using a frequency variation of an external induction coil for applying an electromagnetic field (see U.S. Pat. No. 4,446,562, 1984), a method using an inductance variation of an induction coil (see Japanese Patent Laid-Open Publication No. Heisei 6-122056), etc. An electric load in electromagnetic continuous casting equipment including an induction coil, a casting mold, molten metal, etc. basically varies with the surface level of the molten metal. As a result, there are variations in a voltage from a power supply device to the induction coil and associated current, inductance and frequency. The above methods are adapted to measure the surface level of the molten metal using such a phenomenon. However, the measuring performance is excellent when the surface of the molten metal is within the range of the induction coil, but abruptly degraded when the surface of the molten metal is beyond the range of the induction coil. This requires a band for measurement of the surface level of the molten metal to be limited to the range of the induction coil.
One approach to such a problem is the extension of the surface level measuring band by an auxiliary coil installed above the induction coil. In this approach, however, the measuring band is limited to a range defined by the two coils, too (see Iron and Steel, Vol. 84, p 625, 1998). In a method of measuring an inductance variation of an induction coil using a detection coil, shown in Japanese Patent Laid-Open Publication No. Heisei 6-122056, values measured by the detection coil involve influences resulting from voluntary variations in current to the induction coil with casting conditions. As a result, it is unreasonable to apply the measured values to the accurate measurement of the surface level of the molten metal without correcting such variations.
Another approach is a method using a magnetic field sensor (see Korea Patent Application No. 99-28920), which is applicable to the measurement of the surface level of molten metal in the electromagnetic continuous casting process in that the magnetic field sensor has an excellent sensitivity and a wide detection band. However, a considerable degree of error may occur in the measured result in the case where current to an induction coil is severe in variation.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an apparatus and method for measuring the level of the surface of molten metal in an electromagnetic continuous casting process, which can effectively remove noise components generated from a power supply device of an electromagnetic continuous casting machine and external noise components, accurately measure the surface level of the molten metal and provide molten metal surface level data appropriate to other equipment in accordance with the accurate measurement.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by a provision of an apparatus for measuring the level of the surface of molten metal within a mold in an electromagnetic continuous casting process by detecting a magnetic field applied from an induction coil and an induced magnetic field based on eddy current in the molten metal, comprising power supply means for supplying predetermined AC power to the induction coil and setting a power variable indicative of a variation in the AC power; a detection coil for detecting the sum of the applied magnetic field from the induction coil and the induced magnetic field; amplification/filtering means for amplifying an output signal from the detection coil to a predetermined level and filtering the amplified signal to remove noise components therefrom; and an arithmetic unit responsive to an output signal from the amplification/filtering means and the power variable from the power supply means for detecting the surface level of the molten metal by removing components of the applied magnetic field based on the variation in the AC power to the induction coil from the magnetic field sum detected by the detection coil.
In accordance with another aspect of the present invention, there is provided a method for measuring the level of the surface of molten metal within a mold in an electromagnetic continuous casting process by detecting the sum of a magnetic field applied from an induction coil and an induced magnetic field based on eddy current in the molten metal through a detection coil, comprising the first step of amplifying an output signal from the detection coil to a predetermined level and filtering the amplified signal to remove noise components therefrom; and the second step of determining the surface level of the molten metal by removing components of the applied magnetic field based on a variation in current to the induction coil from the amplified and filtered signal.